I don't have a T23 but, FWIW, I've successfully installed these on a variety of computers going way back to my 1998 Toshiba Tecras 780dvd with 266MHz and 128-192MB RAM (works but only crawls unless doing cli), 2000-ish Thinkpad 600x with 500-850MHz and 576MB RAM (works for basics but a bit too slow for heavier software, merely walking), and 2002-ish Thinkpad A31p with 2.0GHz 2GB RAM (works fine, a nice jog, and faster with the mSATA-IDE adapter), and 2005+ T43p and all the way up to W520 (run great).

Because T23 is a bit older, and probably similar to an A31p era, I'd try AntiX first, it's a very light OS (I use min-jwm desktop for 600x and A31p), MX-16 used XFCE is more fully featured but a bit more resource intensive. Only way to know if it'll work for your T23 is to burn some these live isos and give 'em a try... I hope that this helps.

Puppy Linux should be fine if you use the faster variants of PIIIM Tualatin (1.13ghz, 1.2ghz or 1.26/1.33 if you can find one because they are EXTREMELY RARE) with a gig of RAM.
On the Windows side, W2k is the most responsive and stable one. If you want the newest stuff as possible, try Windows 7 with Firefox 48

Puppy Linux should be fine if you use the faster variants of PIIIM Tualatin (1.13ghz, 1.2ghz or 1.26/1.33 if you can find one because they are EXTREMELY RARE) with a gig of RAM.
On the Windows side, W2k is the most responsive and stable one. If you want the newest stuff as possible, try Windows 7 with Firefox 48

Because of the serious vulnerability risks with old x86 NT, I wouldn't recommend W2K unless it's being used exclusively offline.